The aircraft reg has to be for the State that your licence covers. Most boats carry US reg, some New Zealand.
For you there is no point flying an N or VH reg aircraft if you hold an EASA licence. The aircraft then has to be maintained to the standard of that State.
So ask the boat Capt that you are working from and the engineer that works on the boat.
I have heard of companies just putting a new rego on for convenience that doesn't show up on a register anywhere.